1. Field of the Invention
The present invention relates to a display device using organic electroluminescent (EL) elements.
2. Description of the Related Art
Various types of display devices are known, one of which is a projector type display. This projector type display normally uses a liquid crystal panel. More specifically, a transmitted light is obtained by controlling each liquid crystal element of the liquid crystal panel according to video information and also by controlling the transmitted light by using a polarizing plate. Then, by projecting the transmitted light controlled according to the video information onto a screen, a video image is displayed.
FIG. 11 shows an example of the conventional full-color projector type display. As illustrated, a reflector 104 is disposed behind a light source 102, with the result that a parallel beam of white light is emitted forward. The parallel beam is first reflected by a mirror 106 and is then incident on a dichroic mirror 108, which is arranged tilted 45.degree. from the optical path, and only yellow light is reflected by the mirror 108 in a direction at right angles with the optical path before reflection. Therefore, the transmitted light of the mirror 108 is red light. This red light is reflected in a specified direction by a mirror 110. When the yellow light reflected by the dichroic mirror 108 is incident on a dichroic mirror 112, green light is separated and reflected in a right-angle direction, and blue light passes through the mirror 112. The blue light reflects through mirrors 114 and 116 and goes on to a specified direction. Liquid crystal panels 120, 122 and 124, each including a polarizing plate, are disposed close to the three sides of a dichroic lens 126 and accept red light, green light and blue light, respectively. The liquid crystal panels 120, 122 and 124 cause the red, green and blue light, modulated by the control of the liquid crystals, to fall on the dichroic lens 126. The dichroic lens 126 combines those rays that go into the above-mentioned three sides to form a parallel beam, which is emitted from the one other side. A screen 130 is installed across a lens 128 in front of the one other side of the dichroic lens 126. The red, green and blue rays, which have been modulated separately by the liquid crystal panels 120, 122 and 124, are combined, and a full-color image is displayed on the screen 130.
Among the projector type display devices are audio-visual displays to show a TV screen or the like and head up displays (HUD) used in automobiles or airplanes. In the HUD, light rays that have passed through the liquid crystal panel are projected by means of a dichroic filter or a hologram optical element.
FIG. 12 shows an example of the structure of a conventional dichroic system HUD. In front of a light source 140 formed by a halogen lamp (or a xenon lamp), there is a liquid crystal panel 142. A transmitted light of the liquid crystal panel 142 passes through a shutter 144, a mirror 146 and a collimating lens 148, and is applied to a dichroic filter 150 attached to the inner surface of the windshield glass. Thus, a virtual image of an image modulated by the liquid crystal panel 142 appears in front of the windshield glass.
FIG. 13 shows an example of the conventional hologram system HUD. A light from a light source 160 formed by a halogen lamp (or a xenon lamp) or a CRT (cathode ray tube) is injected into a liquid crystal panel 162 that includes a polarizing plate. The light, modulated by the liquid crystal panel 162, passes through an aberration-corrected lens system 164 and a mirror 166, and is projected onto a hologram optical element (a diffraction grating) 168 provided on the windshield glass. Accordingly, a virtual image of an image formed in the liquid crystal panel 162 is made to appear in front of the windshield glass.
Incidentally, in the display mentioned above, light loses its power greatly when it passes through the liquid crystal panel. To produce color images, three primaries are generated from white light emitted from a light source, and the rays of three primaries are transmitted through the three liquid crystal panels, respectively. In this case, the utilization factor of light is 10% or so. Therefore, to produce bright images, it is required to irradiate the liquid crystal panel with light at a high luminance of about 10000 cd/m.sup.2. Normally, a metal halide lamp or a halogen lamp is used as the light source. A high voltage needs to be used for the light source, and in this case, there is a problem that high heat is generated by the light source. To obtain an adequate transmitted light, it is necessary to irradiate the liquid crystal panel with light with sufficient parallelism. To this end, it is necessary to provide the light source with a large reflector, which is a hindrance to the movement toward weight and size reductions of the device itself. Another problem is the need to provide a large cooling fan for the light source.
With regard to the HUD in automobiles or airplanes, there are requirements to decrease the size, weight and energy consumption. Those problems with the projector type display have been obstacles to the widespread use of HUD.
In the area of devices for information display, there are CRT, the liquid crystal display, the plasma display, the inorganic EL display and so on. However, these kinds of displays are designed to provide a wide angle of field, and make it possible for the viewers to look at the displayed information from a diagonal direction. On the other hand, with some displays, it is desired that the angle of field be limited. In the liquid crystal displays, which are not spontaneous luminescent type displays, the angle of field can be narrowed easily, but there is a problem that it is difficult to achieve an image display with high luminance. With display on the instrument panel of a car, it is required that an information display should be visible to the driver only and that high luminance should be provided to secure enough visibility even when there is direct sunlight coming into the car. Therefore, in view of the above requirements, the liquid crystal display device was not preferable. There are displays that use LED or a fluorescent indicator tube (a magic eye), but they are only capable of simple display, and are not suitable for display with high resolution.